Thermal time delay switch



1959 E. c. WIDMER I 2,915,608

THERMAL TIME DELAY swrrcn Filed Oct. 1; 1958 INVENTOR EMIL C. WIDMER HISATTORNEY to. the glass envelope.

THERMAL THVIE DELAY SWITCH Emil C. Widmer, East Orange, N.J., assignorto Curtiss- Wright Corporation, a corporation of Delaware ApplicationOctober 1, 1958, Serial No. 764,673

8 Claims. (Cl. 200-122) This invention relates to thermal time delayswitches of the type using bi-metal elements associated with a heaterfor opening and closing switch contacts according to predetermined timeresponse, and has for its principal object to provide an improvedthermal time delay switch of this character having co-acting contactstructure arranged so as to ensure substantially constant pressurebetween the contacts throughout the period of contact engagement.

A further object is to provide an improved switch of the above type thatis reliable and efficient in operation .unassembled spring mountingstructure of one of the contacts in Fig. 1, Fig. 3 is a perspective viewof the contact structure of Fig. 1 in its normal operative position, andFig. 4 is a side view of the contact structure shown in Fig. 2.

The switch assembly of Fig. 1 is normally enclosed in an evacuatedglassenvelope or the like, either with or without special gases for improvingcontact operations. The envelope is not shown as it forms no part of thepresent invention. The assembly comprises essentially a supporting base1 that may comprise an insulating disk 2 that is mounted .in a sealingring 3 for joining the base The insulating disk 2 receives the conductorterminals 4 that support the main switch assembly and connect theco-acting contacts and the bimetal heater to suitable external circuits.

- The main switch assembly comprises two bi-metal elements 5 and 6formed as elongated rectangular strips that are secured at the base endsto an insulating spacer block 7 in clamped relation thereto as indicatedby the through bolt 8 and washers 9. Certain of the terminal conductorssuch as 4b are suitably connected to the bimetal elements 5 and 6 forcompleting the circuit between the switch contacts which are mountedrespectively on the bi-metals as presently described. The bimetal 5 isprovided with a heater coil 10 that is mounted in insulating arrangementaround the bi-metal element according to usual practice. The heater issuitably connected at 10a to the terminal conductors 4a as indicated.

The switch contacts 11 and 12 are carried by the bimetal elements 5 and6 respectively. The contact 11 is rigidly mounted at the upper free endof the bi-metal 5 by adjustable means such as a screw fastening 13constituting the electrical connection with the bi-metal 5. Theco-acting contact 12 is resiliently mounted on the bi-metal 6 so that ithas limited following movement with the contact 11.

The resilient mounting structure for the contact 12 United States Patent0 is designed so as to maintain substantially constant pressure betweenthe contacts throughout their travel of contact engagement. To this end,the co-acting contact mounting structure comprises a spring steel strip13 that is suitably secured to the bi-metal 6 at 14 (as by riveting),said spring strip being formed as an elongated compressed loop thatextends approximately half the distance from the securing rivet 14toward the supporting base including the spacer stack at 7. The springloop 13 also forms the electrical connection between contact 12 andbi-metal 6. A detent 15 having an overhanging lip 16 is secured to theupper free end of the bi-metal 6 (and may conveniently comprise asindicated an extension of the strip 13) for engaging and restraining thefree end 13a of the strip. The loop in this position tends to expandbecause of the stressed condition of the strip 13.

Figures 2, 3 and 4 more clearly illustrate the spring strip arrangementfor ensuring limited travel of the contact 12 with respect to itssupporting bi-metal 6. Figures 2 and 4 show the strip 13 in itsunstressed and unassembled condition. The portions of the strip 13b andforming opposite sides of the loop are shown as bent to formapproximately an angle of 70 in the unstressed condition. When the stripis to be assembled, the portions 13!) and 130 are forced toward eachother, with the tapered portion 13c stressed slightly also laterally sothat the tip end 13a slides underneath the lip 16 of the detent in amanner analogous to closing of a safety pin as clearly shown in Fig. 3.It will thus be seen that the spring loop tends to maintain followingengagement of the contacts 11 and 12 as the bi-metals are warpedincidental to heating of the coil 10 and changes in ambient temperature,the latter being compensated by the bi-metal 6.

The present invention is especially adapted for controlling certaintypes of precision circuits wherein variations in resistance at thecontacts cannot be tolerated. For maintaining the contact resistancesubstantially constant, the contacts 11 and 12 are maintained atsubstantially constant pressure engagement throughout. their contactingperiod. This is accomplished by the special configuration of the springstrip 13 which, in its preferred form extends from the secured part at14 a short distance at an acute angle, as indicated at 13d, from thebi-metal 6 and then extends (as portion 13a) generally parallel and inspaced relation to the bi-metal for a distance approximately half thedistance in the example shown between the secured part at 14 and thebi-metal base. However it should be understood that the length of thespring loop is not necessarily related to the length of the supportingbi-metal. The spring strip at the low portion of the loop is bentbackward so that the portion 130 forms with portion 13b an elongatedcompressed loop as shown. The resiliency of the loop at its lowerportion is relatively greater than that of the portions 13b and 130 byreason of reinforcing means, such as ridges 13a pressed in the aforesaidside portions. The ridges 13e as shown extend longitudinally along thecenter axis of said portions for a distance depending on the requiredspring characteristics of the contact mountmg.

The operation of the thermal time delay switch will now be brieflydescribed: assuming first that the heater coil 10 is energized, theresulting heat causes warping of the bi-metal 5 toward the right so thatcontact 11 engages contact 12. As the bi-metal 5 continues to Warp inthis direction the contact 11 moves the contact 12 also toward the rightso that the prestressed spring loop is further tensioned, the travel ofthe engaged contacts away from the detent lip 16 depending on the heaterand the characteristics of the bi-metal 5. Ambient temperature 3 changesare compensated by the bi-metal 6 which is adapted to warp in the samedirection as the bi-metal for a given temperature change, therebytending to maintain the distance between the free ends of the bi-metalsapproximately constant.

Upon deenergization of the heater the bi-metal 5 begins to cool andthereupon to warp slowly toward the left with the contact 12 infollowing engagement with contact 11 until the free loop end 13a engagesthe re straining detent lip 16. During this following engagement thecontact pressure remains substantially constant due to the configurationof the spring mounting above described. When the travel of contact 12 isblocked, further cooling of the bi-metal 5 causes separation of thecontacts and opening of the control circuit according to the prescribedtime delay.

It should be understood that this invention is not limited to specificdetails of construction and arrangement thereof herein illustrated, andthat changes and modifications may occur to one skilled in the artwithout departing from the spirit of the invention; for example thespring loop 13 may be mounted if desirable, on the bimetal 5 having theheater coil, the relative movement of the contacts being essentially thesame in both instances; also if desired, the device may be readilyadapted to have the contacts normally closed, rather than open as shown.That is the bi-metals are simply turned around so that with the heatercold and the contacts positioned so as to engage, the bi-metals warptoward the left rather than toward the right as shown, upon increase intemperature to open the contacts.

What is claimed is:

1. A thermal time delay switch comprising a supporting base, a firstbi-metal element and a second bi-metal element, said elements arrangedso as to extend generally in the same direction in spaced relation toeach other and each element being supported on said base at one end sothat the free ends are moveable relative to each other, a heater elementadjacent said first bi-metal element arranged to cause warping of thefree end thereof relative to the free end of said second bi-metalelement, said second bi-metal element being influenced primarily byambient temperature so as to warp in the same direction as said firstelement for a given change in ambient temperature, a first switchcontact mounted on one of said bi-metal elements adjacent the free endthereof so as to be positioned therewith, a co-acting switch contactcarried by the other bi-metal element, and means for mounting saidco-acting contact on said other bimetal element for relative movementthereto comprising a resilient member secured at one part thereof tosaid other bi-metal element and arranged to form a loop extending fromthe secured part toward said base, detent means secured to the free endof said other bi-metal element and adapted to be engaged by the freeloop end of said member so as to limit the expansion of said loop,

said free end of the loop member carrying the co-acting contact inalignment with said first switch contact so that said contacts areadapted to be in following engagement for a limited travel as determinedby the position of said detent, said loop arrangement being adapted toensure substantially constant pressure between said contacts during saidfollowing travel incidental to relative warping movement of the bi-metalelements.

2. A thermal time delay switch as specified in claim 1 wherein theresilient member forming the loop is a spring strip extending a materialdistance from said secured part toward the base.

3. A thermal time delay switch as specified in claim 2 wherein thespring strip is offset at an acute angle from said other bi-metalelement adjacent the secured part.

4. A thermal time delay switch as specified in claim 1 wherein theresilient member forms an elongated compressed loop extendingapproximately half the distance from the secured part toward the base.

5, A thermal time delay switch as specified in claim 1 wherein thedetent and resilient member are formed as a continuous spring stripforming a closed loop of the safety pin type, said strip being attachedto said other bimetal element at the aforesaid secured part.

6. A thermal time delay switch as specified in claim 2 wherein thespring strip extends for a short distance from the secured part at anacute angle from the strip and then extends generally parallel to thestrip in spaced relation thereto for a material distance towards thebase, thereupon looping in reverse direction to engage and be restrainedfrom further expansion by an overhanging lip of said detent, therebyproviding for relative movement of the co-acting contact with respect tosaid other bimetal elements and restraint of following movement of thecontact with respect to the first contact.

7. A thermal time delay switch as specified in claim 2 wherein thespring strip at opposite sides of the loop is provided with reinforcingportions extending longitudinally of the strip and terminating short ofthe end of the loop so as to increase relative resiliency at said loopend.

8. A thermal time delay switch as specified in claim 7 wherein the loopportion extending from the secured part toward the loop end is offset inspaced substantially parallel relation to said other bi-metal element.

References Cited in the file of this patent UNITED STATES PATENTS2,329,119 Jacobs Sept. 7, 1943 2,658,975 Zuckerman Nov. 10, 19532,754,392 Benedik July 10, 1956 FOREIGN PATENTS 651,051 Great BritainMar. 7, 1951

